If mod_autoindex is installed and enabled, files and directories will be listed whenever you're browsing a directory and no DirectoryIndex files (normally configured as index.html, index.htm, index.php, welcome.html) are present in that directory.

It's a very useful feature but perhaps for security reasons you might not want to allow this. You can disable this feature by using any of these methods:

The easiest way is probably to disable the autoindex module entirely. This would affect all the sites hosted on the server, which could be both good and bad.

The module is called autoindex, and can be disabled by the a2dismod command, as in the following example.

$ sudo a2dismod autoindex

The option could be applied per-directory, via the Directory's Option directive as the following.

<Directory /var/www/mysite>
    Options -Indexes
</Directory>

This could be set in your Apache's configuration file.

The above method would also work by adding a .htaccess file with the following content that you want directory listing to be disabled.

Options -Indexes

Restart Apache for the changes to take effect